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Error messages
How to deal with common error messages, and problems, in HFSS. Out of memory Possible reasons: * The structure is too complicated. Try to simplify it, in a way that will not introduce too much inaccuracy. Keep in mind that high memory usage decreases accuracy, because the mesh cannot be refined as much. If you make a simplification and the results change, it's possible that the "simplified" model is MORE accurate. * The structure includes elements which are too thin. HFSS likes its mesh cells to have similar size in all 3 dimensions. So a large but thin structure will be divided into very tiny mesh cells. A large number of cells will be needed, which will take up too much memory. Often, it's better to replace the thin 3D element with a 2D equivalent. If the thickness of the 3D element was much smaller than the wavelength, often this will not affect the results much. * The computer has too little RAM. Get a new computer! Unfortunate, this usually isn't an option. Also, memory required for simulation increases with the cube (???) of mesh cell dimensions. Doubling the memory will NOT double the size of the structure you can simulate. So try changing the structure instead. Human intelligence works MUCH better here than "brute force". * The frequency is too high. For accuracy, each mesh cell must be smaller than a certain fraction of the wavelength. At high frequency, the mesh cells are too small, the matrix to be solved is too big, and the simulation fails. The last point leads to an important consideration: Structures scale with frequency. A structure that simulates successfully in the megahertz range will also simulate equally well in the gigahertz range - if you make it 1000 times smaller. Generally, a structure should be between roughly 0.5 to 10 wavelengths long at the frequency of interest. If your simulation runs out of memory after a long time (say, half an hour), it's likely that it almost fits in the memory requirements. If it fails quickly (a couple minutes), it is probably WAY too complicated. Out of precision "Discrete solution, process hf3d error: Matrix solver exception: out of precision .." Lack of precision means that the structure is too small relative to the wavelength. When HFSS evaluates the fields at two adjacent points in the structure, the difference is so small that it rounds to zero, breaking the calculation. There are two possible reasons for this: * Your entire structure is too small for the wavelength. This probably means you've accidentally chosen a wavelength which is too big (i.e. GHz not MHz). Or, the whole structure might be too small by orders of magnitude. * One part of the structure is too small. This probably means you left a small gap/offset between two objects that were supposed to touch each other. Look at the structure carefully and try to find this. You can get around this error by choosing your solution frequency (before the sweep) to be much higher than what you want. But that means increased memory usage and less accuracy. HFSS refuses to run simulations you requested Sometimes HFSS does not realize that there are still things left to compute. For example, if you run a successful simulation, and then add a frequency or parameter sweep or choose the "Save Fields" option for simulations, HFSS may think all simulations are finished, and not compute what you want it to. To get around this: Close HFSS, delete the .hfssresults directory, and then run the simulation again. This time it will do everything its supposed to. If you edited a parametric sweep, but HFSS doesn't run it, check that you clicked "Apply" and the results show up in the sweep. The interface is confusing, it's easy to accidentally not save your changes. Simulation runs, but slowly Possible reasons: * Your files are stored over the network. HFSS can create hundreds of megabytes of data for a simulation. Copying all these over the network, and retrieving them when needed, takes a very long time. * You use too much memory. Not enough for the simulation to fail, but enough that it has to swap memory to disk. See "out of memory" error.